(a) Field of the Invention
The present invention relates to an apparatus and method for frequency synchronization, and in particular, to an apparatus and method for completely synchronizing carrier frequency through a digital process in an orthogonal frequency division multiplexing (OFDM) wireless communication system.
(b) Description of the Related Art
In OFDM, original data stream is multiplexed into N parallel data streams each of the data streams modulated with a different frequency using an Inverse Fast Fourier Transform (IFFT), and the resulting signals are transmitted together in the same band.
The OFDM symbols are artificially prolonged by periodically repeating a guard interval which is longer than a maximum channel delay so as to remove the reflections of previous symbols, which preserves the orthogonality such that inter symbol interferences (ISI) and inter channel interference are reduced.
Successful OFDM reception requires that a receiver maintains correct symbol synchronization which means that the receiver knows at which point of time each symbol begins and ends.
Maintaining the symbol synchronization is difficult if a transmitter and receiver are moving with respect to each other. For example, if a mobile station moves around in an urban environment, the propagation path of the signal changes constantly, resulting in attenuation and reflection. Also, the mobile station moving fast causes phase jitter and Doppler shift, resulting in frequency offset which means difference in frequency between the transmitter and receiver. This frequency offset causes inter symbol interference and damages the orthogonality condition required among the subcarriers, resulting in degradation of bit error rate.
Accordingly, in order for the receiver to be able to successfully receive the symbols, the receiver has to synchronize carrier frequency between the transmitter and receiver before performing the Fast Fourier Transform (FFT).
To reduce the carrier frequency offset, a wireless modem adopting the OFDM modulation sends a training signal for channel estimation and initial frequency synchronization. The Wireless Local Area Network standards such as the HIPERLAN/2 developed by the European Telecommunications Standards Institute (ETSI) and the 802.11a of the Institute of Electrical and Electronics Engineers (IEEE) specify a short training sequence in which 16 samples are repeated 10 times and a long training sequence in which 64 time samples are repeated twice.
In the conventional synchronization method, the frequency synchronization device estimates the carrier frequency offsets using the training signals in digital domain and passes the signals through a loop filter so as to control a voltage controlled oscillator (VCO) such that the VCO output is used to synchronize the carrier frequency in analog domain.
In other cases, especially for single carrier transmission, a numerical controlled oscillator (NCO) is used for compensating the frequency offset in the digital domain.
However, in the carrier frequency synchronization method of the conventional OFDM system in which the carrier frequency synchronization is performed in analog domain, it is impossible to perform a precise carrier frequency synchronization since the carrier frequency synchronization is performed using the output of the VCO that is controlled by the signal generated by the estimated frequency offsets and this causes the synchronization time delay.
Also, in the conventional method, since the carrier frequency offset is estimated in digital domain and the frequency offset is compensated using the VCO in analog domain, its implementation and performance analysis are difficult in this mixed analog-digital mode.
As the implementation method for obtaining an arctangent and exponential function for synchronizing carrier frequency in digital domain, the techniques employing the Coordinate Rotation Digital Computer (CORDIC) and Look-up table are used.
However, the CORDIC method has a drawback in that the computing speed is slow and the precision is low despite of its simple implementation. On the other hand, the look-up table method requires lots of memory modules causing a hardware complexity and it is difficult to create addresses in look-up table in spite of its fast speed and high precision.